The larvicidal activities of three sesquiterpenes, alantolactone, isoalantolactone and dihydroisoalantolactone, isolated from the roots of Inula helenium (Compositae) against 3rd and 4th instars of Aedes albopictus (Diptera: Culicidae) and Paratanytarsus grimmii (Diptera: Chironomidae), were examined. The two sesquiterpenes, alantolactone and isoalantolactone, showed LC50 values of 2.7 μg/ml and 11.9 μg/ml for A. albopictus, and 5.1 μg/ml and 4.1 μg/ml for P. grimmii within 48 h, respectively. Alantolactone was significantly more toxic than isoalantolactone against A. albopictus; however, dihydroisoalantolactone did not entirely show lethal effects against the larvae of both species at a concentration of 1,000 μg/ml.